System and method for adjusting the position of a control member of a power plant

ABSTRACT

A system for adjusting the position of a control member of a power plant has a hydraulic fluid operated hydraulic actuator provided with a displaceable member connected to the control member, and having operating parameters; a servo valve configured to control the hydraulic actuator by means of the hydraulic fluid, and selectively operable between at least an open position for allowing fluid connection with the hydraulic actuator and a closed position for blocking fluid connection to the hydraulic actuator; a by-pass fluid device to drain the hydraulic fluid from the hydraulic actuator in any position of the servo valve; and a control device configured to selectively driving the servo valve as a function of an operating parameter of the hydraulic actuator so as to compensate the fluid drain, and keep the operating parameter within a given range about a selected target value.

TECHNICAL FIELD

The present invention relates to a system for adjusting the position of at least a control member of a power plant.

In general, this type of systems comprises at least one hydraulic actuator including a displaceable member connected to the control member, and controllable by a mineral based hydraulic fluid; at least one servo valve, which includes a valve housing, is arranged to control the actuator utilizing said hydraulic fluid, and is selectively operable between an open position for being in fluid connection with the hydraulic actuator and a closed position for blocking fluid connections to the hydraulic actuator; a hydraulic circuit extending through said hydraulic actuator and said servo valve for circulating said hydraulic fluid; and a control device configured to selectively operating the valve in accordance to the requirements of the power plant.

BACKGROUND ART

A system of the above-identified type is often used to adjust the position of the IGV vanes assembly of gas turbines and suffers of a number of problems determined by the mineral based hydraulic fluid.

Mineral based hydraulic fluids are predominantly used in modern heavy duty gas turbines because their use is imposed by regulations. However, these hydraulic fluids are exposed to oxidation and degradation due to temperature load and mechanical stresses. In general, these hydraulic fluids have a tendency to form insoluble contaminants (so called sludge and varnish) that are of polar nature and have a preference to precipitate on polar materials like metals. The softer parts of these insoluble particles (called sludge) will likely attach to metal surfaces easier under given conditions, i.e. a temperature lower than 40° C. and a flow lower than 0.5 l/min. Physical properties of the sludge and varnish are such that the insoluble particles will stay in solution at oil temperatures above at least 40° C. most likely in the range between 42 and 52° C. depending on the make and type of hydraulic fluid. The insoluble particles, like sludge and varnish, affect the operational reliability of servo valves. In many cases the insoluble particles lead to oil flow problems and lock the components of the servo valves. For example, a gas turbine unit which is affected by sludge and varnish may show increasing costs due to trips or other operational disturbances. The problem is considerably relevant for power plants, for example large power producers operating several generator units in peaking mode. In such a case, the mechanical components of servo valves exposed to the hydraulic fluids may become contaminated with sludge and varnish, leading to malfunction of servo valves that hydraulically control generator operations. Hydraulic fluid cleaning methods have been tried to solve the problem, however, so far, they were not completely effective in solving the problem.

In order to solve a similar problem in the field of injection moulding EP 269,091 discloses a hydraulic circuit including a servo valve, and a double effect hydraulic cylinder controlled by the servo valve. The supply branch of the hydraulic cylinder is connected to the hydraulic fluid draining branch of the hydraulic cylinder so as to keep the hydraulic fluid flowing even in absence of stroke of the displaceable member.

The proposed solution is effective when the displaceable member is in abutment against outer elements limiting the stroke of the displaceable member, but it cannot be adopted in a system for adjusting the position of the IGV vane assembly wherein the displaceable member shall assume an indefinite number of adjusting positions without abutments.

DISCLOSURE OF INVENTION

The object of the present invention consists in making a system for adjusting the position of at least a control member of a power plant that can overcome or at least reduce the problems caused by the precipitation of sludge and varnish in the hydraulic fluid.

According to the present invention there is provided a system for adjusting the position of at least a control member of a power plant; the system comprising;

at least one hydraulic actuator operated by a hydraulic fluid, including a displaceable member connected to the control member, and having operating parameters such as the position of the displaceable member and the operating pressure;

at least one servo valve, which includes a valve housing, is configured to control the hydraulic actuator by means of the hydraulic fluid, and is selectively operable between at least an open position for allowing fluid connection with the hydraulic actuator and a closed position for blocking fluid connection to the hydraulic actuator; and

a by-pass fluid device to drain the hydraulic fluid from the hydraulic actuator in any position of the servo valve;

a control device configured to selectively driving the servo valve as a function of at least an operating parameter of the hydraulic actuator so as to compensate the fluid drain, and keep the operating parameter within a given range about a selected target value.

According to the present invention it is possible to keep the hydraulic fluid flowing even when the operational circumstances do not require the circulation of hydraulic fluid through the servo valve and, at the same time, it is possible to keep the displaceable member in a substantially stationary position.

In accordance with a preferred embodiment of the present invention the control device includes a monitoring device and a valve controller; the monitoring device being configured to detect at least one operating parameter, and emit a signal correlated to the operating parameter, whereas the controller is configured to drive the servo valve on the basis of said signal so as to keep the operating parameter within said given range about the selected target value.

In accordance with a further preferred embodiment of the invention the fluid bypass-device has a housing, configured to be detachably connected to the valve housing; the bypassed hydraulic fluid flowing through the housing, which preferably is a metal or metal alloy plate.

According to a still further preferred embodiment of the present invention the housing of the fluid bypass-device and the valve housing are configured to exchange heat with each other.

In this way the hydraulic fluid is kept in motion and is thermally conditioned by the bypass device.

The present invention further relates to a method for adjusting the position of at least a control member of a power plant.

According to the present invention there is provided a method for adjusting the position of at least a control member of a power plant; the method comprising:

adjusting the control member by means of displacing a displaceable member of a hydraulic actuator controllable by hydraulic fluid and having given operating parameters, such as the position of the displaceable member and the operating pressure;

controlling the hydraulic actuator by means of a servo valve including a valve housing, and selectively operable between an open position for being in fluid connection with the hydraulic actuator and a closed position for blocking fluid connections to the hydraulic actuator,

draining the hydraulic fluid from said hydraulic actuator by means of a by-pass fluid device irrespective of the position of the servo valve so as to determine an instability in the operating parameters of the hydraulic actuator;

controlling the servo valve to selectively operating the servo valve as a function of at least one operating parameter so as to compensate the fluid drain, and keep the operating parameter within a given range about a selected target value.

BRIEF DESCRIPTION OF THE DRAWINGS

A non-limiting preferred embodiment of the present invention will be described by way of example in connection with the enclosed figures in which:

FIG. 1 is a schematic view, with part removed for clarity, of a system according to the present invention in a first operational position;

FIG. 2 is a schematic view, with part removed for clarity, of a system according to the present invention in a second operational position; and

FIGS. 3A, 3B, 3C are respectively a bottom view, a top view, and a side view of a component of the system of FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

In FIG. 1 with reference numeral 1 is indicated a system 1 for adjusting the position of at least a control member 2 of a power plant not shown in further details in the enclosed figures. The system 1 comprises a hydraulic circuit 3 for circulating a hydraulic fluid and a control device 4 for monitoring and controlling the operations of the hydraulic circuit 3. The hydraulic circuit 3 comprises a fluid source S; a servo valve 10; a bypass device 20; a hydraulic actuator H controllable by hydraulic fluid, a fluid drain D and a number of lines connecting the above-identified components.

The hydraulic actuator H can be configured in many different ways, and can for example include a double acting linear hydraulic actuator H, a piston/cylinder type device, it can be part of a generator system, a turbine system. In particularly, the hydraulic actuator H can be pressure controlled, wherein an operating state of the hydraulic actuator H can be set or changed by (for example, temporally) adjusting pressure of hydraulic fluid that is fed to respective control ports AH, BH of the hydraulic actuator H. The hydraulic actuator H comprises a displaceable member 5, namely a piston, which is connected to control member 2 and can be displaced in two opposite directions. The hydraulic actuator H has two chambers H1 and H2 arranged on opposite sides of the piston and connected to respective lines L. The displacement of the displaceable member 5 is function of the pressure difference in chambers H1 and H2.

The servo valve 10 is a four way three positions sliding valve and has the function of hydraulically controlling the hydraulic actuator H. In other words, valve 10 is configured to feed hydraulic fluid to the hydraulic actuator H, and to receive hydraulic fluid from the hydraulic actuator H, via respective fluid lines L. The servo valve 10 comprises a housing 11 provided with first fluid ports, namely a supply port P and return (drain) port T; and second fluid ports A, B suitable to be connected to the hydraulic actuator H via the respective fluid lines L. According to an embodiment not shown the servo valve includes a different number of first and/or second ports. The servo valve 10 comprises a valve mechanism 13 namely a slider that is selectively moveable inside the valve housing 11 between a closed position to block fluid connection between the hydraulic actuator H and fluid supply S and fluid drain D as shown in FIG. 1; and two open positions for driving the hydraulic actuator H in two respective opposite directions (one of the two open position is shown in FIG. 2).

The servo valve 10 further includes an adjuster Q operatively connected to the valve mechanism 13 for selectively displacing the valve mechanism 13 inside the valve housing 11.

The control device 4 is configured to adjust the valve mechanism 13 of the servo valve 10, for example based on one or more signals correlated to the operating parameters of the hydraulic actuator H such as the position of the displaceable member 5 and/or the pressure in chambers H1 and H2. The control device 4 comprises a controller C that can be configured in different ways, for example including suitable hardware and/or software, a microcontroller, computer, or in a different manner. For example, the servo valve 10 can include one or more controllable valve adjusters Q, for example an actuator or a servo, that can adjust the operating state of the valve 10, the adjuster Q being (for example electronically) controllable by the controller C. In the present embodiment, the controller C and adjuster Q are depicted as being separate components (the controller C being located outside of the valve housing 11, and the adjuster Q being included in the housing 11). Alternatively, controller and adjuster can be integrated with each other.

Also, for example, the servo valve controller C can be configured to adjust the servo valve 10 (i.e., to adjust the valve mechanism 13), to set the operating parameters of the hydraulic actuator H to a predetermined values To that aim, for example, valve controller C can be configured to control the servo valve 10, depending on one or more actuator related signal, relating to the detected operating parameters of the hydraulic actuator H. The actuator related signal can include various types of signals, for example a control signal, sensor signal, a linear position transducer signal, a feedback signal, a signal relating to a detected functioning of the hydraulic actuator H, and/or a different type of signal. The control device 4 comprises a monitoring device MH associated to the hydraulic apparatus H in order to detect the operating parameters such as the position of the displaceable member 5 or the pressures in chambers H1 and H2. During operation, the monitoring device MH provides the controller C with the signal correlated to at least one operating parameters. As an example only, the signal can relate to a piston position in case the hydraulic actuator H is a piston/cylinder type device, or a parameter that relates to the piston position. Similarly, it can relate to a turbine power output parameter in case the hydraulic actuator H is part of a turbine generator system. In the present embodiment, the controller C and monitoring device MH are depicted as being separate components: Alternatively, controller C and device MH can be integrated with each other.

In FIG. 1, the valve mechanism 13 is set to a neutral state to block fluid connections between the first ports P, T and second ports A, B. The valve mechanism 13 can be adjusted, from the neutral state, to at least one fluid transmission state to allow fluid connections between the first ports P, T and second ports A, B. FIG. 2 shows one fluid transmission state, wherein the supply port P is in fluid connection with port B, and wherein return (drain) port T is in fluid connection with port A. In an alternative fluid transmission state (not shown), supply port P can be in fluid connection with supply port A, and return port T in fluid connection with port B.

The second fluid ports A, B of the valve are (indirectly) in fluid connection with respective ports AH, BH of the hydraulic actuator H. Also, the valve's first fluid ports P, T are in fluid connection with a fluid supply S and a fluid drain D, via respective hydraulic fluid connections s1 and d1.

The hydraulic fluid supply S can be configured in various ways, and may for example include one or more fluid transport lines (i.e., fluid ducts, conduits), one or more fluid pumps, one or more fluid reservoirs, one or more fluid treatment devices, a filter system for filtering the hydraulic fluid, a fluid heating system to heat the fluid to a desired fluid temperature.

The bypass-device 20 is configured to allow fluid bypass-connection between first port T and one of the second port A, B in any operational condition of the servo valve 10 or, in other words, in any operational positions of the valve mechanism 13.

In the present embodiment, the fluid bypass-device 20 comprises a first fluid bypass-connection 33 a between second ports A and the return port T that is connected to a return line d1; and a second bypass-connection 33 b between second ports B and the return port T (via the return line d1).

The fluid bypass-device 20 has a bypass housing 21, which is configured to be detachably connected to the valve housing 11 by an optional fluid line connector plug 45, which is depicted in dashed lines and is provided with end ports of the four fluid lines s1, d2, L leading to the source S, drain D and hydraulic actuator H.

A connection between the bypass housing 21 and a valve housing 11 (and optional plug 45) can be achieved in various ways, for example using one or more attachment devices, clamping devices, and/or suitable interconnection means. The present embodiment is provided with a number of bolts 41, for bolting the bypass housing 21 to the valve housing 11. The valve housing 11 is provided with bolt receivers 42 (configured to cooperate with bolt ends), and the by pass housing 21 include bores 46 to lead the bolts via the device housing 21 to the valve 10. In the depicted operating position (see FIG. 1-2), the device 20 is connected to the valve housing 11, and can provide a fluid connection between drain port T and at least one of the second port A, B. In yet a further embodiment, the bolts 41 can be used to connect the optional fluid line connector plug to the bypass-device 20, as well, using respective bores of the plug.

The bypass housing 21 of the fluid bypass-device 20 and the valve housing 11 can be configured to exchange heat with each other, particularly for thermally conditioning the valve housing 11. Also, optionally, the fluid bypass-device 20 can be configured to be thermally conditioned by a respective bypass-fluid flowing through the fluid bypass-device 20. The by pass housing 21 is substantially a plate in which the first fluid channels (32 a, 32 b), the second fluid channels (31 p, 31 t) and the at least a bypass channel (33 a, 33 b) are made, namely machined. Further, the bypass housing 21 is made of a material having a high thermal conductivity, for example metal or metal alloy and has a temperature conditioning surface 71 that is substantially in thermal contact with a facing surface of the valve housing 11 when the device 20 is connected to the valve housing 11, to exchange heat with the valve housing (particularly via heat conduction).

The bypass housing 21 has two first fluid channels 31 p, 31 t, (in particular bores machined in the bypass housing 21), including a source channel 31 p and return channel 31 t, that are connected with respective first fluid ports P, T of the valve 10 after assembly. The device 20 also includes two second fluid channels 32 a, 32 b (in particular bores machined in the bypass housing 21), and connected to respective second fluid ports A, B of the valve 10. In this embodiment, the first and second channels 31 p, 31 t 32 a, 32 b of the bypass device 20 all extend in parallel, particularly extending normally with respect to two outer surfaces 71, 72 of the bypass housing 21 (see FIG. 3B).

The present bypass-device 20 comprises four first ports p1, t1, a1, b1 (located along housing surface 71) facing the valve housing 11 after mounting, and second ports p2, t2, a2, b2 (located along housing surface 72) facing away from the valve housing 11.

In particular, first ends of the fluid channels 31, 32 of the bypass-device 20 provide a first source port p1, a first return port t1, and two first actuator ports a1, b1, which are connected to respective opposite ports P, T, A, B of the valve, after mounting. Preferably (see FIG. 3C), the bypass-device 20 is provided with sealing means, for example resilient seals, for example O-rings 29 that provide sealed hydraulic connections between the first ports p1, t1, a1, b1 of the bypass-unit 20 and the respective valve ports P, T, A, B.

In the embodiment, the first ports p1, t1, a1, b1 of the bypass-unit 20 are arranged to be in precise alignment with the respective valve ports P, T, A, B, when the device's housing 21 is mounted onto the valve housing 11. As follows from FIG. 3C, for example, the first ports p1, t1, a1, b1 can be located at the corners of a substantially square pattern (the actuator ports a1, b1 being located diagonally with respect to each other, and the source and drain port p1, t1 being located diagonally with respect to each other), in case the valves ports P, T, A, B are be located in such a configuration.

Also, as follows from the FIGS. 1 and 2, second ends of the channels 31 p, 31 t, 32 a, 32 b of the bypass-device 20 provide a second source port p2, a second return port t2, and two second actuator ports a2, b2, which are connected to respective fluid lines s1, d1, L of the system after assembly. In the present embodiment, this connection can be achieved via the optional plug 45 that is provided with end ports of all the (four) fluid lines s1, d2, L leading to the source S, drain D and hydraulic actuator H. Alternatively, the fluid lines s1, d2, L can be connected directly to the second ports p1, t1, a2, b2 of the by-pass device 20, using suitable fluid line connectors.

The second ports p2, t2, a2, b2 of the bypass-unit are arranged to be in precise alignment (i.e. are located in a straight lines with respect to each other) with the respective first ports p1, t1, a1, b1 of the bypass device 20.

In the present embodiment, the bypass device 20 is arranged such, that a bypass fluid flow through the device 20 leads to a respective fluid flow (leakage flow) through the valve housing 11. Particularly (see the drawings), the present bypass-device 20 can be mounted onto the valve ports P, T, A, B to provide fluid communication between these valve ports P, T, A, B and respective first device ports p1, t1, a1, b1; after mounting, the bypass-device 20 provides external device ports p2, t2, a2, b2 that effectively ‘replace’ the valve ports P, T, A, B, for example to receive fluid line connectors or the fluid line connector plug 45.

In particular, the housing 21 of the fluid bypass-device 20 comprises a first fluid bypass channel 33 a to connect the second fluid channels 32 a (associated with actuator ports a1, a2) to the return channel 31 t (and therefore to drain D).

The housing 21 of the fluid bypass-device 20 comprises a second fluid bypass channel 33 b to connect the second fluid channels 32 b (associated with the other actuator ports b1, b2) to the return channel 31 t (and therefore to drain D).

For example, the fluid bypass-channels 33 a, 33 b extend substantially transversally between the respective fluid channels 31 p, 31 t, and 32 a. 32 b (see FIG. 1-2).

The bypass device 20 is controllable to adjust a flow rate of bypass fluid flowing through a respective fluid bypass connection 33 a, 33 b. Preferably, the flow rate can be adjusted over a desired range, for example from zero flow rate to a certain maximum bypass fluid flow rate. In the present embodiment, a channel width or diameter of each bypass channel 33 a, 33 b is adjustable (preferably in a range from zero to a maximum channel width or diameter), to set a respective bypass-flow. For example, the bypass-device 20 can include a bypass-control mechanism 25, operable to adjust bypass flows during operation.

In an embodiment, the bypass-control mechanism 25 can be automatically and/or remotely (for example electronically) controllable, for example by the controller C. In the present embodiment, the bypass-control mechanism is manually controllable, and includes two manually controllable needle valve devices 25 a, 25 b, to control the flow rate through the two bypass-channels 33 a, 33 b. Each needle valve device 25 a, 25 b can be set to a bypass-channel blocking state to close the respective bypass-channel 33 a, 33 b. Each needle valve device 25 a, 25 b can be set to a respective opening state, allowing fluid flow via the respective bypass-channel, 33 a, 33 b, preferably such that the flow rate of the fluid flow can be set thoroughly and accurately.

With reference to FIGS. 3A-3B, the housing 21 support the needle valve devices 25 a, 25 b.

In a further embodiment, the bypass-device 20 is provided with a protecting mechanism, configured to prevent unauthorized control over the bypass-control mechanism 25. For example, the protecting mechanism can include a blocking mechanism that can block operation of the control parts 25, or a covering that can be locked onto the bypass-device 20 to prevent handling of the control parts 25.

Operation of the embodiment can include a method to thermally condition the valve 10. During operation of the present hydraulic system, the pressure of hydraulic fluid in a supply line s1 (of the fluid supply S) that is upstream with respect to the valve 10 (and bypass-device 20), is higher than the pressure of hydraulic fluid in a downstream return line d1 that leads (from return port t2 of bypass-device 20) to the drain D.

In use, the servo valve 10 is driven by adjuster Q on the basis of a control signal emitted by controller C in order to position the control member 2 in accordance with the operational requirements of the power plant. The controller C is in connection with a control unit (not shown) of the power plant not shown in the enclosed Figures. For example the control member is a IGV vanes assembly or a member connected to the IGV vanes assembly.

When the control member 2 has to be set in a selected target position, controller C provides for setting the displaceable member 5 in a selected target position. When the displaceable member 5 reaches the selected target position, the servo valve 10 is set in the closed position and the hydraulic fluid does not flow through the hydraulic circuit 3.

According to the present invention the bypass-device 20 determines a leakage in the hydraulic circuit 3 from the hydraulic actuator H towards the drain D and an instability in the position of the displaceable member 5 and the control member 2. The displacement of the displaceable member 5 from the selected target position is detected by the monitoring device MH, which emits a signal, and the controller C emits a signal as function of the displacement signal for the adjuster Q in order to displace the valve mechanism 13 from the closed position to an open position in order to restore the position of the displaceable member in the selected target position. In this way, the hydraulic fluid circulates in the hydraulic circuit and stagnation of the same is prevented in particular in the servo valve 10.

The controller C is provided with a given admissible range of displacement of the displacement member from a target position. The range is selected so as not to impair the operation of the power plant and keeping as continuous as possible the flow of the hydraulic circuit through the hydraulic circuit 3. The higher the flow of the hydraulic fluid, the lower the precipitation of insoluble particles.

The same control process of the servo valve 10 can be implemented by selecting as operating parameter the pressure in the hydraulic cylinder H in particular the differential pressure in cambers H1 and H2 of the hydraulic cylinder H. In this case the monitoring device MH provides for detecting a first pressure in the first chamber H1 of the hydraulic actuator H; emitting a first signal correlated to the first pressure; detecting a second pressure in the second chamber H2 of the hydraulic actuator H; and emitting a second signal correlated to the second pressure. The controller C provides for emitting a third signal correlated to the differential pressure between the first pressure and the second pressure; and controlling the servo valve 10 on the basis of the third signal so as to keep the differential pressure in the hydraulic actuator H within a given range about the selected target value. The selected target value allows keeping the displaceable member 5 in the selected target position or at least in a given acceptable range about the selected target position.

Further the control process can be implemented using both position signal and pressure signal.

In other words, the bypass device induces a change in the operating parameters of the hydraulic actuator H. The change is automatically counteracted by the control device 4, by setting the valve 10 into a respective fluid transmission mode (as in FIG. 2), such, that high pressure fluid from the source S cancels (and even temporarily reverses) a fluid pressure drop experienced at the hydraulic actuator H due to the fluid bypass. Particularly, the valve 10 is controlled such that pressures in the hydraulic actuator H control lines L are restored to stationary pressure values that provide the desired (for example predetermined) operating parameters.

As a result, the system can experience a continuous fluid bypass-flow, flowing through the valve 10 and bypass-device 20, when the actuator H is in a desired (for example stationary) operating condition to perform a respective actuator function, without the servo valve 10 being in a neutral state. The bypass flow flows through the valve 10, and bypasses the actuator H (i.e. the bypass flow does not particularly flow to and from the actuator's fluid ports AH, BH).

Thus, advantageously, the system 1 provides to keep the hydraulic fluid flowing through the servo valve 10 and to thermally condition the servo valve 10 using hydraulic bypass fluid flow. For example, to that aim, a temperature of the hydraulic bypass fluid (supplied from the source S) can be higher than 40° C., particularly higher than 50° C.

Besides, as in the present embodiment where the valve mechanism 13 is adjustable between a neutral state and one or more fluid transmission states, the bypass fluid flow can lead to the valve mechanism 13 being in a fluid transmission state during a predetermined (for example stationary) operating state of the hydraulic actuator H.

In the present embodiment, heat is supplied to the servo valve 10 utilizing warm hydraulic fluid, particularly by setting the valve in a fluid transmission state. Also, as follows from the above, in case the servo valve 10 is a servo valve, operation of the system can include a method to thermally condition a servo valve. For example, the valve controller C can adjust the valve mechanism of the servo valve depending on one or more signals relating to a working condition of the hydraulic actuator H. Alternatively, according to an aspect of the invention, operation can then include supplying heat to the valve when the valve is in its neutral valve state. For example, heat can be supplied utilizing dedicated heating means, for example one or more electrical heating devices (not shown) integrated with or mounted on the valve housing 11. For example, the heater can be configured to heat the valve housing to a temperature higher than 40° C., for example at least 50° C.

The present bypass-device 20 can create a defined hydraulic fluid flow through the valve component and the associated hydraulic system under various operating conditions. For example, the hydraulic fluid flow can thermally condition various components of the system, specifically in case the system is controlled to operate discontinuously (for example, in case the controller C controls the servo valve 10 to maintain a certain valve state during a substantial part of an operating period). According to a further embodiment, during operation, the servo valve 10 is controlled to maintain a certain valve state during a large operational period of at least one hour, particularly at least several hours, more particularly at least 24 hours. For example, the servo valve 10 can be controlled to maintain a certain valve state during at least 99% of a total operational life-time of the valve (i.e., most of the time, the servo valve 10 holds a certain desired operative valve state, to hydraulically control an hydraulic actuator H that is coupled to the valve 10). Then, preferably, the bypass-device 20 is set to ensure that a hydraulic bypass-flow (‘leakage’ flow) flows through the servo valve 10 during such a long period, to thermally condition the valve. In this way, precipitation of certain oxidation products (such as sludge and varnish) in the hydraulic system and its components can be prevented surprisingly well.

Even though the present description refers explicitly to a double acted hydraulic cylinder, the present invention is applicable to single acted hydraulic cylinder counteracted by an elastic member such as a spring: this type of hydraulic actuator are often used to adjust the position of a control member in power plants.

Although the illustrative embodiments of the present invention have been described in greater detail with reference to the accompanying drawings, it will be understood that the invention is not limited to those embodiments. Various changes or modifications may be effected by one skilled in the art without departing from the scope of the claims. 

1. System for adjusting the position of at least a control member (2) of a power plant; the system (1) comprising; at least one hydraulic actuator (H) operated by a hydraulic fluid, including a displaceable member (5) connected to the control member (2), and having operating parameters such as the position of the displaceable member (5) and the operating pressure; at least one servo valve (10), which includes a valve housing (11), is configured to control the hydraulic actuator (H) by means of the hydraulic fluid, and is selectively operable between at least an open position for allowing fluid connection with the hydraulic actuator (H) and a closed position for blocking fluid connection to the hydraulic actuator (H); a by-pass fluid device (20) to drain the hydraulic fluid from the hydraulic actuator (H) in any position of the servo valve (10); and a control device (4) configured to selectively driving the servo valve (10) as a function of at least an operating parameter of the hydraulic actuator (10) so as to compensate the fluid drain, and keep the operating parameter within a given range about a selected target value.
 2. The system as claimed in claim 1, wherein the control device (4) includes a monitoring device (MH) and a valve controller (C); the monitoring device (MH) being configured to detect at least one operating parameter, and emit a signal correlated to the operating parameter, whereas the controller (C) is configured to drive the servo valve (10) on the basis of said signal so as to keep the operating parameter within said given range about the selected target value.
 3. The system as claimed in claim 1, wherein the servo valve (10) includes a valve mechanism (13) that is moveable in the valve housing (11) in the closed position to block fluid connections with the hydraulic actuator (H), and at least in an open position to allow fluid connections with hydraulic actuator (H).
 4. The system as claimed in claim 1, wherein the fluid bypass-device (20) has a housing (21), configured to be detachably connected to the valve housing (11); the bypassed hydraulic fluid flowing through the housing (21), which preferably is a metal or metal alloy plate.
 5. The system as claimed in claim 4, wherein the housing (21) of the fluid bypass-device (20) and the valve housing (11) are configured to exchange heat with each other.
 6. The system as claimed in claim 4, wherein said housing is configured to be thermally conditioned by the hydraulic fluid bypassed through the fluid bypass-device (20).
 7. The system as claimed in claim 4, wherein the bypass device (20) is controllable to adjust a bypass flow flowing through the housing (21).
 8. The system as claimed in claim 4, comprising a hydraulic fluid supply (S) and a hydraulic fluid drain (D); wherein the servo valve (10) includes a number of first fluid ports (A, B) that are in fluid connection with the hydraulic actuator (H); and a number of second fluid ports (P, T) that are in fluid connection with a fluid supply (S) and a fluid drain (D); the fluid bypass-device (20) being configured to define a fluid bypass-connection in at least one of the fluid connections between the servo valve (10) on the one hand and the hydraulic actuator (H), and drain (D) on the other hand.
 9. The system as claimed in claim 8, wherein the bypass device (20) comprises a number of first fluid channels (32 a, 23 b), connected to respective first fluid ports (A, B) of the valve (10) and a number of second fluid channels (31 p, 31 t), connected with respective second fluid ports (P, T) of the valve (10); and at least one bypass channel (33 a, 33 b) connecting a first and a second fluid channel (32 a, 32 b, 31 t); the first fluid channels (32 a, 32 b), the second fluid channels (31 p, 31 t) and the at least bypass channel (33 a, 33 b) are made into the valve housing (21).
 10. The system as claimed in claim 9, wherein the bypass-channel (33 a, 33 b) includes a control valve (25 a, 25 b) to control a flow rate through the bypass-channel (33 a, 33 b).
 11. The system as claimed in claim 10, wherein the first and second channels (32 a, 32 b, 31 p, 31 t) of the bypass device (20) mutually parallel.
 12. The system as claimed in claim 4, wherein the bypass device housing (21) comprises first fluid ports (p1, t1, a1, b1), facing the valve housing (11), and respective second fluid ports (p2, t2, a2, b2) facing away from the valve housing (11).
 13. Method for adjusting the position of at least a control member (2) of a power plant; the method comprising: adjusting the control member (3) by means of displacing a displaceable member (5) of a hydraulic actuator (H) controllable by hydraulic fluid and having given operating parameters, such as the position of the displaceable member (5) and the operating pressure; controlling the hydraulic actuator (H) by means of a servo valve (10) including a valve housing (11), and selectively operable between an open position for being in fluid connection with the hydraulic actuator (10) and a closed position for blocking fluid connections to the hydraulic actuator (H), draining the hydraulic fluid from said hydraulic actuator (H) by means of a by-pass fluid device (20) irrespective of the position of the servo valve (10) so as to determine an instability in the operating parameters of the hydraulic actuator (H); controlling the servo valve (10) to selectively operating the servo valve (10) as a function of at least one operating parameter so as to compensate the fluid drain, and keep the operating parameter within a given range about a selected target value.
 14. The method as claimed in claim 13 including detecting the position of the displaceable member (5); emitting a signal correlated to the actual position of the displaceable member (5); and controlling the servo valve (10) on the basis of said signal so as to keep the position of the displaceable member (5) within a given range about the selected target position value.
 15. The method as claimed in claim 13 including detecting the pressure in the hydraulic actuator (H); emitting a signal correlated to said pressure; and controlling the servo valve (10) on the basis of said signal so as to keep the pressure in the hydraulic actuator (H) within a given range about a selected target value.
 16. The method as claimed in claim 15 including detecting a first pressure in a first chamber (H1) of the hydraulic actuator (H); emitting a signal correlated to said first pressure; detecting a second pressure in a second chamber (H2) of the hydraulic actuator (H); emitting a second signal correlated to said second pressure; emitting a third signal correlated to the differential pressure between the first pressure and the second pressure; and controlling the servo valve (10) on the basis of said third signal so as to keep the differential pressure in the hydraulic actuator (H) within a given range about the selected target value. 